Лако-красочные материалы — производство

The term ‘volatility’ is usually taken to refer to the speed at which a material evaporates. It is not an exactly defined property, and no universally accepted standards are laid down within the scientific literature. As implied above, the ‘dry-down’ or evaporation behaviour of even an unsophisticated perfume on a simple solid substrate, such as a paper smelling strip, is complex. Some materials are so volatile that they are lost much more rapidly than other components (within minutes), while materials at the opposite end of the volatility spectrum may remain for a considerable time (weeks or months). However, there is no doubt that when these very different types of molecule are present in the same mixture (as is the case for the majority of perfumes), ingredient interactions modify the evaporation behaviour to some extent, and, for those materials with intermediate molecular properties, the constitution of the perfume can govern their behaviour.

An understanding of the inherent tendency of an ingredient to escape into the gas phase is a useful starting point when considering perfume volatility. To a first approximation, the relative molecular mass (RMM) and the boiling point of a perfume ingredient provide some guidelines to behaviour. For materials for which boiling point data are not available, it is generally a sound alternative to look at chromato­graphic behaviour. For example, the retention time for a material to elute through a gas chromatographic column containing a non-polar phase is often strongly related to boiling point (in fact, such columns are commonly referred to as ‘boiling point’ columns).

Let us examine the relationship between boiling point and molecular size more closely. Table 11.1 comprises physicochemical information on a number of materials that are or have been used in the fragrance industry. The data were drawn from a number of sources, and some of the parameters (e. g. ‘log P’ and ‘sp which are described later) were calculated from specific mathematical models, so that slightly different

values may be found in the literature. However, in the context of volatility and substantivity, the emphasis is not on absolute values but rather on understanding and quantifying the differences between molecules.

The RMM of limonene (1), the major terpene in citrus oils, is 136, while its boiling point is ca. 178 °С at atmospheric pressure. Contrast this with Cervolide® (12-oxacyclohexadecanolide, a macrocyclic musk; 2), the RMM of which is 256 and boiling point is over 290 °С. The majority of perfume ingredients fall between these two extremes, although there are, of course, exceptions. Many odorous materials are more volatile than limonene and find some use in perfumes. Examples of these are methyl butanoate and its isomer ethyl propanoate, with boiling points of 102 °С and 99 °С respectively, and both with RMMs of 102. (These ingredients appear in a few perfumes, but are much more widely utilized in the flavour industry.) Odorous materials with RMMs greater than that of Cervolide® are much rarer since there appears to be a natural molecular size limit above which the human nose cannot detect, corresponding to around 300 RMM (cf. odorous steroids), presumably because volatility becomes too small.

Perhaps a more direct way to assess volatility is to look at the saturated vapour pressure of an ingredient. Saturated vapour pressure refers to the equilibrium pressure exerted by a substance in a closed system at a specified temperature (the volume of the system must, of course, be greater than that of the substance). Table 11.1 again gives representative values. Consider, for example, the volatile material 1,8- cineole (3), which is utilized in many ‘fresh’ perfumes and is also commonly found in toothpaste flavours. This material has a vapour pressure of ca. 2mmHg at 25 °С (similar to that of limonene), which in the context of the perfumery world is very high. Most musks have vapour pressures that are three to five orders of magnitude smaller than that of cineole. Vapour pressure is directly related to the mass present in the gas phase, so the fact that musks are perceivable at all to the
human olfactory system is a tribute to the impressive ‘dynamic range’ of olfaction (the sensitivity of canine olfactory systems is even better!).

Equation (1) is a useful route to calculating headspace concentra­tions above a pure substance from vapour pressures; c is the gas phase concentration in g 1_1, p° is the saturated vapour pressure in mmHg, and T is the temperature in Kelvin. Equation (2) is the same equation restated in terms of concentration m in mol l-1 at a temperature of 25 °С for cases where the molecular mass is unknown. These equations derive directly from the ideal gas equation.

c = 0.01604 (/) (i? MM)/(273.2 + T) (1)

logio(m) = logio(/>°) — 4.269 (2)

So far, we have dealt with pure materials. When liquid mixtures are considered, the headspace composition reflects the constitution of the liquid phase. Each component of the mixture is present in the gas phase, but its concentration depends on the nature and concentration of the other components. Clearly, when an ingredient is incorporated into a liquid mixture, the same amount is no longer present in the headspace (assuming that it is truly diluted, i. e. that the system is homogeneous with no phase-separated droplets). The saturated vapour pressure still gives a useful guide to the concentration in the headspace, as is evident from equation (3), where p is the partial vapour pressure of the ingredient, x its mole fraction in the liquid and p° its saturated vapour pressure. The parameter у is known as an activity coefficient, and may be considered as an indicator of non-ideal behaviour.

p = yxp° (3)

When у is unity, equation (3) reduces to the ‘ideal’ form known as Raoult’s Law, in which the partial pressure of a component above a homogeneous liquid system is directly proportional to its mole fraction. It is very similar to an empirical expression first formulated by Henry for solutes in dilute solution (originally for gases in liquids), in which the solute partial pressure is proportional to concentration [equation (4), where c is the concentration in g 1— 1 and H is referred to as the Henry’s Law constant].

p = He

By re-writing equation (4) in terms of mole fraction (and adjusting the

dimensions of the constant H), the similarity to Raoult’s Law can be seen. In fact, the main difference lies in the choice of standard states for the definition of у or #, but this is beyond the scope of this chapter. More importantly, we need to understand how and why у varies from unity. The differences are driven by the interactions that take place between the various components of the mixtures, discussed in the next section.